linux-kernel - Re: [linus:master] [x86] 4817f70c25: stress-ng.mmapaddr.ops_per

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Message-ID: <488401565cac6b5f9e3232d0ca481055876c919b.camel@surriel.com>
Date: Wed, 29 Jan 2025 11:14:29 -0500
From: Rik van Riel <riel@...riel.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Qi Zheng <zhengqi.arch@...edance.com>, Peter Zijlstra
 <peterz@...radead.org>,  David Hildenbrand	 <david@...hat.com>, kernel test
 robot <oliver.sang@...el.com>, 	oe-lkp@...ts.linux.dev, lkp@...el.com,
 linux-kernel@...r.kernel.org, Andrew Morton	 <akpm@...ux-foundation.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>, Catalin Marinas <catalin.marinas@....com>, David
 Rientjes	 <rientjes@...gle.com>, Hugh Dickins <hughd@...gle.com>, Jann Horn
	 <jannh@...gle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Mel
 Gorman	 <mgorman@...e.de>, Muchun Song <muchun.song@...ux.dev>, Peter Xu	
 <peterx@...hat.com>, Will Deacon <will@...nel.org>, Zach O'Keefe	
 <zokeefe@...gle.com>, Dan Carpenter <dan.carpenter@...aro.org>, "Paul E.
 McKenney" <paulmck@...nel.org>, Frederic Weisbecker <frederic@...nel.org>,
 Neeraj Upadhyay	 <neeraj.upadhyay@...nel.org>
Subject: Re: [linus:master] [x86] 4817f70c25: stress-ng.mmapaddr.ops_per_sec
 63.0% regression

On Wed, 2025-01-29 at 16:12 +0000, Matthew Wilcox wrote:
> On Wed, Jan 29, 2025 at 10:59:20AM -0500, Rik van Riel wrote:
> > Below is a tentative fix for the issue. It is kind of a big hammer,
> > and maybe the RCU people have a better idea on how to solve this
> > problem, but it may be worth giving this a try to see if it helps
> > with the regression you identified.
> 
> Perhaps better to do:
> 
> +++ b/mm/page_alloc.c
> @@ -97,8 +97,8 @@ static DEFINE_MUTEX(pcp_batch_high_lock);
>   * On SMP, spin_trylock is sufficient protection.
>   * On PREEMPT_RT, spin_trylock is equivalent on both SMP and UP.
>   */
> -#define pcp_trylock_prepare(flags)     do { } while (0)
> -#define pcp_trylock_finish(flag)       do { } while (0)
> +#define pcp_trylock_prepare(flags)     rcu_read_lock()
> +#define pcp_trylock_finish(flag)       rcu_reada_unlock()
>  #else
> 
>  /* UP spin_trylock always succeeds so disable IRQs to prevent re-
> entrancy. */
> 
> with appropriate comment changes
> 
Agreed. Assuming this change even works :)

Paul, does this look like it could do the trick,
or do we need something else to make RCU freeing
happy again?


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